Refineries employ atmospheric and vacuum distillation towers to separate crude oil into narrower boiling fractions. These fractions then are converted into fuel products, such as motor gasoline, distillate fuels (diesel and heating oils), and bunker (residual) fuel oils. Some of the low boiling fractions from various units of the refinery are directed to petrochemical plants, where they are further processed into highly refined chemical feedstocks to be used as raw materials in the manufacture of other types of products, such as plastics and basic chemicals.
Within the petrochemical plant, processing of low boiling, mixed olefin streams primarily derived from pyrolytic cracking of hydrocarbons often require that the stream be treated in a caustic scrubber to remove acid gases, such as hydrogen sulfide and carbon dioxide. A caustic scrubber is a vessel containing an aqueous solution of caustic (NaOH, KOH, etc.) through which liquid or gaseous hydrocarbons are passed and mixed to wash out or "scrub out" the acid gases and impurities from the hydrocarbon stream. The hydrocarbon stream entering the caustic scrubber also may contain aldehydes and ketones, their precursors, such as vinyl acetate, or other impurities, that are hydrolyzed or otherwise converted to aldehydes and salts of organic acids in the highly alkaline environment of a caustic scrubber. Such compounds will herein be referred to as "reactive compounds." These reactive compounds either (a) contain carbonics, or (b) form carbonyls under highly alkaline conditions, that are susceptible to classic aldol condensation reactions. Carbonyls that are susceptible to classic aldol condensation reactions hereinafter will be referred to as "reactive carbonyls."
Under highly alkaline conditions, lower molecular weight aldehydes, such as propionaldehyde (propanal) and especially acetaldehyde (ethanal), readily undergo base catalyzed aldol condensation at ambient temperatures. The result is the formation of oligomers and polymers which precipitate out of the scrubbing solution as viscous oils, polymeric gums, and solids. These precipitates can foul the processing equipment and result in the reduction of processing throughput and costly equipment maintenance or repair.
In the past, organic reducing agents or organic and inorganic oxidizing agents have been proposed to prevent such polymerization. These organic agents might successfully retard polymerization in caustic scrubbers; however, the organic agents also tend to undergo other reactions which can reduce their effectiveness as aldol condensation inhibitors. Also, most of the oxidizing and reducing agents in current use only react with reactive carbonyls at a molar ratio of about 1:1 at maximum efficiency. A fewer number of these compounds can only reduce a maximum theoretical ratio of 2 moles of a reactive carbonyl per mole of the inhibitor compound. As a result, a relatively large amount of oxidizing or reducing agent must be added to retard polymerization.
A more effective and economical method of retarding aldol condensation in caustic scrubbers would be highly desirable.